Dizzyingly Fast-Spinning Stars Slow Down by Flying Apart

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The spectacularly fast-whirling dead stars known as millisecond
pulsars put the brakes on their spinning in large part by
blasting pieces of themselves into space, a new model suggests.

Pulsars are the super-dense, strongly magnetized cores of massive
stars left behind after they
go supernova. Specifically, pulsars are neutron
stars made of densely packed neutrons, with each sugar
cube-size piece of neutron star matter weighing as much as a
mountain of about 100 million tons.

These neutron stars can bulk up on matter and energy by
cannibalizing companion stars, a process that makes the pulsars
give off X-rays and spin extraordinarily fast — at rates of 100
to 1,000 turns each second — earning them the moniker " millisecond
pulsars."

Later, after siphoning all the matter in the outer envelopes of
their companion stars, millisecond pulsars gradually slow down
and emit radio waves instead. However, little was known about
what happens during the actual slowdown before these pulsars
start blasting radio waves.

A new study by astrophysicist Thomas Tauris at the University of
Bonn in Germany may have the answer. Tauris found that
millisecond pulsars can hit the brakes dramatically. In the end
stages, the dead stars can lose more than half their rotational
energy, the study found.

Computer models suggest that the magnetospheres, or shells of
charged particles around millisecond pulsars, grow as their
companion stars shrink. This growth exerts a braking torque on
the pulsars. In addition, when matter from the companion stars
enters these magnetospheres, it can get blasted away instead of
glomming onto the pulsars, which also helps slow the pulsar's
spinning.

"Now we know that the very same process responsible for spinning
up old neutron stars to extraordinary fast spin rates with
periods of 1 to 10 milliseconds is actually also causing the
millisecond pulsars to spin down again," Tauris said.

"An important implication of this work is that the X-ray-emitting
millisecond pulsars, which are still accreting material from
their companion star, should, on average, spin faster than the
millisecond pulsars emitting radio waves," Tauris added. "This is
exactly what the current observational data seems to suggest."

Tauris detailed his findings in the Feb. 3 issue of the journal
Science.

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